Polonium-210 is found in mainstream cigarette smoke and it has been suggested that the alpha radiation from the 210Po deposited in the lung is etiologically related to bronchogenic carcinoma. Our study now supports previoulsy reported data in the literature that the 210Po activity in the lower lung is higher in smokers than in nonsmokers. This study will also establish the amount of 210Po in the bronchial tree of smokers and nonsmokers. The concentration on the bronchial tree, although small, is thought to be nonuniform and could contribute a significant alpha dose to critical basal cells in bronchial epithelium. We have developed an experimental technique to determine the distribution of alpha activity on the bronchial tree using cellulose nitrate film as a nuclear track detector. With a knowledge of the quantity and microdistribution of 210Po it should be possible to determine if the 210Po alpha dose is comparable with past human exposures to alpha emitters in the lung known to be carcinogenic.